Python: Add `security-severity` for `py/insecure-cookie`

Matching the Java query
7d4767a4f5/java/ql/src/Security/CWE/CWE-614/InsecureCookie.ql (L7)

python/ql/src/experimental/Security/CWE-614/InsecureCookie.ql

@@ -4,6 +4,7 @@
  *              interception.
  * @kind problem
  * @problem.severity error
+ * @security-severity 5.0
  * @id py/insecure-cookie
  * @tags security
  *       external/cwe/cwe-614